Field-glass-type, night-vision observation visualization systems are frequently used by foot soldiers for night missions. A night-vision field-glass comprises a light-intensifying tube to amplify the residual light of a dimly lit observation scene. This residual light originates from the light of the moon or the light of the stars. Conventional night-vision goggles with light intensification comprise a lens focusing the beams on a photocathode which converts light into electrons, an electronic amplification stage, a phosphor screen and an optical projection system picking up the image formed on the phosphor screen comprising one or two eyepieces. This optical system is possibly folded. The image intensifier or IL conventionally uses a second-generation tube, i.e. comprising an “S20” or “S25” multi-alkali photocathode or a third-generation intensifier with an AsGa or AsGaP photocathode. The responses of the photocathodes cover the visible spectrum with a slight extension towards the near infrared.
The applicant previously filed a French patent application with publication number FR 2 863 718 on Dec. 16, 2003. This application describes a folded optical architecture of a light-intensifying monocular field-glass. Moreover, this document discloses an embodiment with two eyepieces enabling fusion of the intensified image with a video image formed on a screen. Light-intensifying field-glasses intended to be worn directly by a user for night missions are of unit magnification.
A light-intensifying field-glass provides the observer with a faithful perception of natural images. The human eye easily becomes accustomed to images of this type. However one of the disadvantages of field-glasses of this type is the observation of a scene in a condition of very low luminosity, for example inside buildings or during nights with no moon or with an overcast sky. The residual light becomes too weak for the night-vision visualization system to be effective.
To solve this problem, vision field-glasses exist comprising an image sensor sensitive to the temperature of objects. Unlike an IL tube, which performs the function of both image sensing and its display, an infrared detector is merely an image-capturing instrument. The imaging system differs greatly according to the spectral band concerned.
The visible and near infrared bands produce images which originate from the reflection of ambient light on objects, the light source being the sun, moon or stars, etc. The reflectivity of the objects depends greatly on the incident wavelength: Contrast inversions may then occur when the wavelength changes. This property can be exploited to detect objects hidden against a background, by comparing the responses given in different spectral bands.
In the 0.4 μm-2 μm domain, two bands must be taken into consideration. The first region extends approximately from 0.4 μm to 1 μm, which coincides more or less with the sensitivity band of silicon. The SWIR band between 1.3 μm and 2 μm and meaning “Short Wave Infra Red” is the band where the vibration state changes of the OH radical known as “Night Glow” of the high layers of the atmosphere converge. Sensors sensitive in this band are essentially based on InGaAs technologies, although other technologies based on MCT or Ge up to 1.8 μm are also available. These detectors are therefore more effective in overcast weather when the light from the moon or stars is reduced.
Passive imaging based on thermal infrared follows a totally different principle. It detects heat sources. An ambient light background is no longer required. A plurality of spectral bands are possible: Far infrared, known by the name “LWIR” meaning “Long Wave Infra Red”, covering the 8-12 μm spectrum and its sub-bands, and medium infrared, known by the name “MWIR” meaning “Medium Wave Infra Red”, covering the 3-5 μm band, which mixes actual transmission mechanisms and solar reflection. The MWIR band is suitable above all when very hot sources need to be detected, whereas the LWIR spectrum is intended more for the observation of objects at ambient temperature.
Infrared devices are particularly useful for applications in which hot objects hidden by vegetation need to be detected. However, a thermal image presents a disorienting appearance for the observer, quite different from the rendering of visible or near infrared images.
A second important point must be considered. As previously mentioned, a light-intensifying tube combines two functions in a single component:                Image sensing and amplification;        The display of the image on a phosphor screen.        
The image formed on the phosphor screen cannot simply be transformed into an electronic video signal. It can only be observed using an optical projection system. In the 0.4-2 μm band, other image-sensing technologies based on CMOS or CCD sensors provide access to a video signal. They cannot perform the second function, i.e. the display function, without an auxiliary module. Compared with light-intensifying tubes, these sensors are generally less sensitive and have a lower resolution and a higher energy consumption. In the infrared band, the image display function requires an auxiliary screen.
Sensor-fusion solutions exist to solve these problems. The American patent application US 2007/0084985 is known from the prior art. This document describes an optical fusion device comprising an intensified channel and an infrared channel visualized on a screen and a camera. It involves a monocular device. The device comprises four channels, an image-sensing channel for the intensified channel and an image-sensing channel for the infrared channel, a display channel for a recording camera, and also for the projection eyepiece. The intensification device can be used with or without the camera module of the display channel in the embodiment with the independent camera module, as described in paragraph 14 of the text of the description. The camera module can then be connected by means of a connector. This device has the disadvantage of being monocular only. Furthermore, it enables fusion only with a predetermined IR camera which is not adaptable according to requirements. This system also consumes energy when the intensified channel and the IR channel are used simultaneously. An energy storage module connected to an electronic system is used by the night-vision device to power all the functional components.
The American patent application US 2008/0302966 is also known from the prior art. This document describes an intensified-channel, night-vision device which can be connected to an IR camera. Any type of IR camera can be used and can easily be connected to the night-vision system by an attachment means using clips. According to this fusion system, two intensified channels are present, and the image of the screen of the IR channel is projected onto a single intensified channel. Consequently, the fused image appears on a single output channel and the night-vision system can be used only in monocular use for a fused view mode.
The patent EP 1857854 is known from the prior art, describing modular-architecture, panoramic, night-time goggles comprising an online light-intensifying optical module, an “HUD” (“Head Up Display”) module and a camera module. According to this architecture, a light-intensifying module can be complemented by the HUD module to fuse the intensified images with information from an external system. The function of the camera is to record the images composed of intensified images and information originating from the HUD. These goggles present a modular architecture but do not allow the fusion to be carried out with the intensified channel and the IR channel.